Cost estimation interior finishing / new construction

Hello community,

next year it could be time for us. Plot 500m² with a slight slope in Heilbronn now available. Over the last 5 months, we have obtained numerous offers from various construction companies based here in Heilbronn. From prefabricated to Ytong kit houses, solid construction, and architects.

House should look like this:

Dimensions 8.9 x 10.7
Net floor area 226m²
Living area 139m²
Knee wall 1.40
Gable roof 35 degrees
WU basement
Monolithic masonry (optional)
KfW 70
Heat pump with underfloor heating
Decentralized exhaust system (optional)
Fireplace (optional)
Walk-in shower
Upper floor windows 1.10m high
2 half-turned reinforced concrete stairs

Offers from basically all turnkey construction companies are around €270,000 with about €10,000 tolerance.
For ancillary costs, we planned the average value from the offers of €32,000 and painting/flooring as own work about €10,000. Building with architect and prefabricated house are ruled out.

Total costs for moving in approx. €312,000

Now, in the last 3 weeks, we have received 2 offers upon request for the shell including basement, roof covering, drainage system and the services incurred up to that point (earthworks, building application, surveying, etc.) €133,000

Now to my actual question. What would the interior finishing cost approximately if from that point on you manage all trades yourself? Electrician, window maker / doors / sills, tiler, and plasterer are available within the family circle. But the rest is missing.

My roughly studied calculation so far :-)

Roof structure insulation €3,000
Heat pump with underfloor heating €25,000 Viessmann Vitocal 200a
Ventilation €9,000
Sanitary + equipment €20,000
Electrical €10,000 Jung AS 500
Windows €15,000
Screed €8,000
Plastering €15,000
Floor coverings €10,000
Doors / window sills €5,000

Additional ancillary costs
Sewage, inspection shaft, utility connections €18,000


Total costs for moving in approx. €271,000

Does the calculation add up or is it wishful thinking?

Thanks in advance!!

I believe the interior finishing is roughly fairly priced; plastering and painting work are missing - I find 133K € for the white tank and complete shell construction very reasonable. Should the basement be developed? How many bathrooms, which category of flooring? Depending on the answer, it might be tight...

The basement remains a utility cellar. 1x bathroom 13m², 1x guest WC 3m², kitchen 15m² will be tiled. The rest will be covered with parquet. Material costs per m² are currently estimated at approx. 50€.

But then the adhesive that you still need for underfloor heating is missing.

Screed 8000€

Screed only costs about 15 euros per sqm. But plumbing is quite tight with 20,000, the 5,000 will be gone quite quickly.